composite, whereas when they are not alike, it is composite. Yet this is indisputably not the case.

Moreover, on this assumption, the parts differ in essence. If they differ in essence, it does not follow that whatever is possible for one is possible for the other. Rather, they cannot be equal with respect to what is necessary, possible, or impossible for them. For if each necessarily took the place of the other, they would be alike, whereas the assumption is that they differ. This is a contradiction. It is therefore clear that, if the parts are indivisible and differ in essence, they cannot take one another’s place. Consequently, the claim that they must be capable of separating is invalid. Rather, one may say: If their separation entails their taking one another’s place, then they cannot separate while differing in essence, because things with different essences cannot take one another’s place.

This applies if by “parts” he means indivisible atoms, which is what the unqualified reference to their separation suggests. If, however, by “parts” he means large parts, the ninth argument may be stated: The meaning is that it is composed of large parts such that, if they differ, each of them must be divisible into indivisible parts. You have said, “They must be either alike or different.” You may then be told: Let us assume that they are alike; and although they are alike in their attributes,